1. Field of the Invention
This invention relates to processes intended to modify the characteristics of hydrocarbons of high molecular weight such as are found in heavy oils. In particular, the present invention relates to processes intended to reduce the viscosity and specific gravity of heavy oil. Specifically, the present invention is related to processes intended to increase the volume of light hydrocarbons distilled from a heavy oil feedstock at a selected temperature.
2. Background of Related Art
Crude oil is a non-uniform, highly complex mixture of hydrocarbon compounds (combinations of carbon and hydrogen atoms) with varying amounts of sulphur, nitrogen, oxygen, and other impurities. The composition of crude oils can vary considerably, even in nearby oilfields. For example, crude oil adjacent the Kern river in Kern County, Calif., U.S.A., has an API gravity of 12.6, a sulphur content (in percent by weight) of 1.19, a specific gravity of 0.982, and a viscosity (SSU at 100.degree. F.) of 6000 seconds; all at a depth of 1,099 to 1,183 feet. Alternatively, crude oil adjacent Greeley in Kern County, Calif., U.S.A., has an API gravity of 37.2, a sulphur content (in percent by weight) of 0.31, a specific gravity of 0.839, and a viscosity of 41 seconds; all at a depth of 11,260 feet to 11,500 feet.
From a non-technical viewpoint, heavy oil can be described as crude oil with a consistency similar to that of cold molasses. However, a technical description indicates that heavy crude oil has a lower hydrogen-to-carbon ratio than lighter crude oil. Because carbon atoms are about twelve times heavier than hydrogen atoms, the density (weight per unit volume) of heavy crude oil is greater than that of lighter crude oil--hence the name, heavy oil.
High specific gravity (which is related to density) and viscosity are properties of heavy oil that cause major production and handling problems. Viscosity is the resistance of fluid to flow.
Although there was no precise definition of heavy crude oil in the past, the definition adopted by the U.S. Department of Energy for its former pricing regulations (and the definition most often used by the petroleum industry) was any crude oil with an API gravity of 20.degree. or less.
Recently, a more precise definition has been adopted. Heavy oil is any crude oil with an API gravity ranging from 10.degree. to 20.degree. (inclusive) at standard conditions and with a gas-free viscosity ranging from 100 to 10,000 centipoises (inclusive) at original reservoir temperature. Tar sand oil, also known as bitumen or ultra heavy oil, is any crude oil with an API gravity less than 10.degree. and a gas-free viscosity greater than 10,000 centipoises.
Crude oil is a mixture of many different chemical components. Each component has its own boiling point; therefore, each component theoretically can be separated from the mixture through distillation. The problem however with heavy oil is the difficulty and expense entailed in increasing the volume of light hydrocarbons distilled from a heavy oil feedstock. Typically, this is done by increasing the hydrogen-to-carbon ratio. This can be accomplished by either removing carbon or by adding hydrogen. Carbon is typically removed by coking, solvent deasphalting, or catalytic cracking. Hydrogen is typically added by hydrotreating or hydrocracking. Other refining processes are discussed in Leffler, William L., "Petroleum Refining for the Non-technical Person", Tulsa, Okla., Petroleum Publishing Company (1979) and Nelson, W.L., "Petroleum Refinery Engineering", New York, McGraw-Hill, pp. 75-77 (1969).
Hydrocracking processes are known which utilize a catalyst in a hydrogen environment to convert heavy distillates into lighter distillates such as gasoline or jet fuels. As discussed further below, such processes typically include adding to the heavy oil feedstock or distillate a source of donor hydrogen such as hydrogen gas. Unfortunately, typical heavy-oil feedstocks have relatively high metal content (100 parts per million or higher) thus limiting the application of hydrocracking because the metals contaminate the catalyst.
There are several issued patents related to the field of the present invention.
U.S. Pat. No. 3,830,730 relates to a method for improving the viscosity of hydrocarbon lubricating oil fractions. The method uses a solid-bed absorbant, liquid cyclohexane at 50.degree. to 300.degree. F. as an eluent, a hydrogenation catalyst and hydrogen gas at pressures between 750 and 5,000 psi.
U.S. Pat. No. 4,399,025 relates to a solvent extraction process for re-refining used lubricating oil. This patent involves use of tetrahydrofurfuryl alcohol (THFA) in a solvent extraction operation to remove impurities, the use of sub-atmospheric pressures (10 to 100 mm Hg absolute) and temperatures of about 300.degree. F. in a steam-stripping operation to recover the THFA for recycling.
U.S. Pat. No. 4,434,045 relates to a process for converting petroleum residuals. The process uses gaseous hydrogen at partial pressures ranging from 1500 to 2500 psi and temperatures ranging between 800.degree. and 850.degree. F.
U.S. Pat. No. 4,462,893 relates to a process for producing pitch for use as raw material for carbon fibers. The '893 process uses various organic chemicals for solvent extraction at temperatures ranging from 734.degree. to 842.degree. F.
U.S. Pat. No. 3,968,023 relates to a method of upgrading residual oils using various organic compounds for solvent extraction and hydrogen partial pressures ranging from 800 to 3,000 psi.
U.S. Pat. No. 4,487,687 relates to a method of processing heavy hydrocarbon oils. The method of this patent involves use of coke as a deasphalting agent prior to hydrogenation, the use of recycled oil as a hydrogen donor solvent at approximately a 1:1 weight ratio to the feedstock, and the use of pressures ranging between 60 and 170 atmospheres during hydrogenation.
U.S. Pat. No. 4,292,168 relates to a method of upgrading heavy oils by non-catalytic treatment with hydrogen and a hydrogen-transfer solvent. The method of this patent uses hydrogen-transfer solvents at a weight ratio ranging from 0.2 to 3.0 of the feedstock weight, temperatures ranging from 608.degree. to 932.degree. F. and pressures ranging from 20 to 180 atmospheres.
U.S. Pat. No. 3,083,155 relates to the use of steel enclosures having a hydrogen partial pressure of 100-700 lbs/square inch and temperatures of 550.degree. to 1100.degree. F.
Notwithstanding the disclosures in the above patents, there is a continuing need for an efficient process for reducing the specific gravity and viscosity of heavy oil. There is a further need for a process for increasing the volume of light hydrocarbons distilled from a heavy oil feedstock at a selected temperature. There is a need for a process to accomplish these objectives which operates (1) at low pressures (near atmospheric pressure), (2) without an external hydrogen gas supply, (3) without being dependent upon a solvent extraction process, and (4) which utilizes a small amount of an active reagent.